1. Field of the Invention
The invention relates to an ink-droplet ejecting apparatus of inkjet type.
2. Description of Related Art
There is known an inkjet printer as an ink-droplet ejecting apparatus, which includes an inkjet head that may be of the type including a plurality of ink passages which are defined in the head and each of which includes a pressure chamber and ends at one of a plurality of nozzles open in a surface of the head. The head also includes a plurality of piezoelectric actuators provided for the respective pressure chambers. To eject droplets of ink from each nozzle, an electrical drive signal in the form of pulses forming a specific waveform is applied to each of the actuators to deform the actuator, thereby pressurizing the ink in the pressure chambers to eject ink droplets as desired.
When a pulse is applied to each actuator, a pressure wave occurs in the ink in the corresponding pressure chamber, and propagates along the ink passage. The time that the pressure wave occurring in the pressure chamber takes to propagate one way along the ink passage, or in a longitudinal direction of the ink passage, from one of opposite ends of the ink passage to the other end thereof, will be referred to as a one-way propagation time AL. For instance, the ink passage may extend from a common ink chamber to a nozzle via the pressure chamber. In this case, an end of the ink passage is at the nozzle, and the other end of the ink passage is at one of the opposite ends of the common ink chamber on the side of the nozzle. However, when the pressure chamber and the nozzle are connected to each other via a thin communication hole or the like, the one end of the ink passage may be at an end of the thin communication hole or the like on the side of the pressure chamber, and when the pressure chamber and the common ink chamber are connected to each other via a thin connecting passage or a restricting portion, the other end of the ink passage may be at an end of the thin connecting passage or restricting portion on the side of the pressure chamber. To maximize the energy efficiency of an ink-droplet ejection and the volume of the ejected ink droplet, a pulse width of the pulse is made the same as the one-way propagation time AL.
Meanwhile, an inkjet printer performs recording of an image on a recording medium, typically by ejecting toward the recording medium ink droplets of various volumes to print dots of various sizes, or recording areas, on the recording medium. In other words, the volume of each droplet corresponding to one dot is required to be changeable or selectable. For instance, a waveform of a drive signal for printing a single dot is determined to be a series of a plurality of pulses, so that the single dot is formed by a plurality of ink droplets, or so that a part of an ink droplet beginning to get off of the nozzle is pulled back to reduce the printed dot. Further, in some cases, a stabilizing pulse or a cancelling pulse is applied subsequent to a main pulse that is for ejecting an ink droplet, in order to suppress or damp a vibration or pulsation remaining in the ink after the ejection of the ink droplet from adversely affecting the following ejection.
JP-B2-3551822, which is publication of a patent granted for the present applicant, discloses a way of increasing the volume of an ink droplet, or the size of a dot. That is, a first ink droplet is initially ejected, but before the first ink droplet completely gets off, or leaves, the nozzle, ejection of a second ink droplet is initiated, so that a single larger ink droplet formed by coalescence of the two ink droplets is ejected onto the recording medium. More specifically, according to a technique disclosed in the publication, a drive signal includes a first pulse, a second pulse as a main pulse, and a third pulse, that are sequentially applied in this order to constitute one set of pulses. A pulse width of the main pulse (or the second pulse) is the same as, and synchronized with, a one-way propagation time T (corresponding to the above-mentioned one-way propagation time AL) of a pressure wave, and a pulse width of the first pulse is 0.35 T-0.65 T. The third pulse is applied to a purpose other than for ejecting an ink droplet, and a pulse width of the third pulse is relatively small. Thus, the first pulse is initially applied in order to eject a first ink droplet at low energy efficiency, but before the first ink droplet completely gets off a nozzle, the second pulse is applied to eject a second ink droplet at high energy efficiency to form a coalescent ink droplet of a large volume, Then, the third pulse is applied in order to damp a residual component of the pressure wave in the ink passage.
On the other hand, there are known three ways of decreasing the volume of an ink droplet. A first way is that a pulse width of the main pulse (or the second pulse) of the drive signal is made different from the one-way propagation time AL in order to purposely lower the energy efficiency of the ink droplet ejection, thereby reducing the volume of the ink droplet. A second way, which is disclosed in JP-A-11-170515, is that a first ink droplet is initially ejected, but when the first ink droplet partially gets off the nozzle, a second pulse is applied at a timing to pull back the first ink droplet, thereby reducing the volume of the ink droplet. The third way of decreasing the volume of an ink droplet is disclosed in JP-A-11-227203 (see especially FIG. 2 and paragraphs 0027 and 0028), where a main or ejection pulse, a volume-reducing pulse, and a stabilizing pulse are applied in this order in a single cycle, and the driving of the head is performed at a frequency of 10 kHz.
However, when the volume of an ink droplet is to be decreased by either of the above-described methods, the speed at which the ink droplet is ejected (which may be referred to as “ejection speed” hereinafter) lowers. When the two methods are employed in combination in order to considerably decrease the volume of an ink droplet, the ejection speed further lowers. The lowering in the ejection speed deviates the landing position of the ink droplet, i.e., the position of the printed dot on the recording medium, from a desired position. That is, the decrease in the ejection speed lowers the accuracy in the landing position of the ink droplet.
Recently, there has been a demand for enhancing the recoding rate of the inkjet printers, in turn demanding to enhance the frequency of the driving the actuators. That is, a drive cycle time for forming one dot has been required to be decreased. In a technique where the ejection pulse is applied at the beginning of each drive cycle time, like the technique disclosed in the above-mentioned publication JP-A-11-227203, a single pulse, namely, the ejection pulse, should generate sufficiently great energy to eject a droplet. Hence, a pulse width of the ejection pulse is required to be synchronized with the pressure wave occurring in the ink in order to generate a great pressure by superimposing the ejection pulse on the pressure wave. Further, the stabilizing pulse for damping the great pressure should be applied with a sufficiently large interval from the ejection pulse, in a sufficiently large pulse width. Hence, an entire length of a single drive signal including a plurality of pulses becomes relatively large, thereby making the drive cycle time long and making it impossible to enhance the recording rate.
To shorten the drive cycle time, it is necessary to shorten the one-way propagation time AL, which is a time taken by the pressure wave caused in the ink upon a deformation of the piezoelectric actuator to propagate one way along the ink passage, and which is a factor determining the pulse width of each of the plural pulses of one drive cycle time. Although this can be achieved by decreasing a length of the ink passage including the pressure chamber, the decrease in the length of the ink passage involves decrease in the pressure chamber, resulting in increase in a drive voltage applied to the piezoelectric actuator in order to produce the ejection pressure of the same level as in the past. However, the increase in the drive voltage is limited.